Use an InfraRed Non-contact thermometer to check the balance of multi-transistor power amplifiers. A couple of simple modifications to the thermometer that will allow easy use.

Power Transistors' Worst Enemy...Heat!
While trouble shooting a solid state amplifier which had power transistor failure, I found it is difficult to determine if all the transistors in a multi-pill amplifier are contributing on an equal basis. This is especially true when it is very impractical to meter the individual drain (or collector) currents. In the final analysis it is the operating temperature level of each transistor that becomes especially critical. It occurred to me that I could key-up the amplifier with enough drive for safe operation and monitor the case temperature. The thermometer being used is the Extech Mini IR Thermometer model EXT42520 and was purchased from Tucker Electronics, Dallas Texas (see inset...Click to enlarge).

The IR Unit Does the Job
I found that my non-contact IR thermometer could do this job but it wasn't practical to hold the thermometer in position, push the "read" button and key the transmitter all at the same time. I decided to modify my thermometer for a standard photo tripod mount and to design a simple "robo-finger" to key the thermometer on a continuous basis. I started to install a 9 vdc input jack on the thermometer, but found that the battery drain for the test time involved was not a serious problem. I decided to operate using the internal 9 vdc battery and omitted this modification.

Fascinating and Useful Device

The IR Non-contact type thermometer is a very fascinating and very useful measuring instrument. Though, keep in mind that it only measures surface temperature and there are many things that can affect your readings. In this case I was primarily looking for equality of case temperature... not absolute temperature. The following photos and captions should provide you with complete modification instructions. Of course many variations can be designed to meet differing circumstances.

The Tripod Attachment assembly is fabricated from a thick brass strip. The holes are drilled for the bottom of the thermometer and the 1/4-20 nut is sweated on with soft solder.

The Attachment assembly is then painted black to match the thermometer and tripod. Matching holes are drilled into the bottom of the thermometer to match suitable attaching screws.